1. Field of the Invention
The invention relates to a calender arrangement.
2. Description of the Background Art
In vertical calenders having a plurality of rolls, one of the intermediate rolls is provided with a main drive and transmits the torques to all the other rolls frictionally, by means of tangential forces, from one roll to the next and via the web to be treated and the roll covers. This leads to undesired horizontal deflections, that is to say to deformations of the intermediate rolls. In addition, the web structure and the roll covers are impaired. FIG. 1 shows a schematic side view of a known off-line calender, in which an intermediate roll 2, that is the drive roll with a main drive, is driven.
In an on-line calender, before the paper web is threaded, and with the nips open, all the rolls in contact with the paper web are accelerated by their auxiliary drives, and the drive roll with the main drive is accelerated to a rotational speed at which the respective circumferential speed corresponds to the web speed of the paper, which can be 1000 m per minute and much higher. After the nips have been closed, power is introduced only by the main drive of a single drive roll. This is illustrated schematically in FIG. 2, in which the main drive 4 is represented by a large drive symbol and the auxiliary drives 6 are represented by a small drive symbol. In FIGS. 1 and 2, the horizontal deflections at the centre of the roll are drawn schematically as a centre offset.
In modern calenders, the top roll and the bottom roll of the roll stack are designed as controllable-deflection rolls. Because of their internal frictional losses, necessitated by their functioning, these two rolls need a greater drive force than the remaining calender rolls. In addition, the most extreme deformation of the paper web takes place in the first nip. In conventional calenders having a main drive, these two power components have to be dragged through the entire roll stack and, in so doing, stress the paper web and the roll covers in a damaging way as a result of frictional transmission of the tangential forces. This results in increased wear of the roll surfaces and a reduction in the service life of the rolls.
According to an earlier proposal in DE-196 50 576.3, all the rolls are provided with their own (power) drives, and the powers of the individual drives are coordinated with one another in such a way that the horizontal roll deformations are minimized.